1. Field of the Invention
This invention relates to natural gas refueling systems, and more particularly, to a system and method for compressing natural gas and for refueling motor vehicles.
2. Description of Related Art
The use of compressed natural gas ("CNG") as an alternative fuel for motor vehicles is well known. Natural gas is in most cases a less costly and cleaner-burning fuel than gasoline. One disadvantage of natural gas as a motor vehicle fuel is the volume required to store the quantity of gas needed to provide a range of travel comparable to that experienced with gasoline. In order to store a sufficient volume of natural gas to provide a reasonable range of travel, it has been thought desirable to compress the natural gas to a pressure of about 3000 to 3600 psi or higher.
Because the vehicle tank pressures needed to store sufficient natural gas to provide a reasonable range of travel are relatively high when compared to available consumer line pressures, the refueling of vehicle storage tanks presents yet another problem. Refilling vehicle storage tanks with CNG within a time period comparable to that required to refill conventional vehicle fuel tanks with gasoline can necessitate the use of large, expensive, multistage compressors. Alternatively, home or on-board CNG refueling systems have been developed that can deliver the compressed gas at the required pressure, but such systems are characterized by very low flow rates, necessitating long periods (such as overnight) for refueling.
Among the various systems for refueling vehicle storage tanks that have previously been disclosed, one conventional system uses a large, multistage compressor to compress the natural gas to about 4000 psi or greater and then holds the CNG in large volume intermediate storage tanks at that pressure. During refueling, the CNG is allowed to flow into the vehicle storage tanks until the vehicle tank pressure is about 3000 psi. After refueling, the intermediate storage is replenished with sufficient gas to again raise the storage pressure to about 4000 psi. This system is inefficient because of the repetitive need to charge storage tanks to about 4000 psi.
Another system, disclosed in U.S. Pat. No. 4,646,940, utilizes a differential pressure measuring apparatus in controlling CNG refueling. The patent discloses preferentially refueling the CNG tanks of a vehicle first from low pressure, then intermediate pressure, and finally, high pressure storage tanks. A reference cylinder at 2750 psi is used to cut off the refueling operation.
U.S. Pat. No. 4,501,253 discloses a low volume (approximately one cubic foot per minute) on-board automotive methane compressor for refilling vehicle storage tanks by compressing the gas from available line pressure to about 2000 to 3000 psi.
U.S. Pat. Nos. 4,515,516 and 5,169,295 disclose systems in which liquid pressure is used to boost CNG pressures in a storage/refueling process. U.S. Pat. No. 4,515,516 discloses a home use natural gas refueling system in which a liquid is used to boost the gas from line pressure to greater than 2000 psi. The system utilizes a variable rate pump which pumps the compression fluid at a high rate for low pressures and a low rate for high pressures. (An illustrative flow rate is about one gallon per minute of compression fluid above 600 psi.)
U.S. Pat. No. 5,169,295 discloses a higher volume liquid-based compression system that can be mounted on a car, truck, boat, train or plane, but is preferably mounted on a tractor trailer truck with the hydraulic pumps connected to the tractor engine by a transfer case. The maximum pressure of the liquid supplied from the liquid supply means is less than the minimum pressure of the gas from the gas-supplying conduit. In the preferred embodiment the supply pump has an maximum output pressure of about 350 psig, and the maximum pressure of the gas-supplying conduit may range from about 400 to about 2900 psig. Illustrative pumping rates for the compression liquid range up to about 200 gpm.
Other previously disclosed CNG refueling systems utilize adsorbent-filled cylinders to reduce the tank pressure needed to store a predetermined amount of natural gas. Such systems are disclosed, for example, in U.S. Pat. Nos. 4,522,159; 4,531,558; and 4,749,384.